It is very common to feel weak after a fever. To help your body recover and combat work fatigue, look for a daily multivitamin containing B-complex vitamins (especially B12 and B6) for energy metabolism, Vitamin C and Zinc to support immune recovery, and Vitamin D3 and Magnesium for muscle strength. Take one tablet daily after a meal. Along with this, focus on staying well-hydrated and eating a balanced diet, as supplements work best alongside proper nutrition. If your weakness persists beyond a couple of weeks, please consult a doctor for a routine evaluation.